The overall objectives of this proposal are to characterize lymphocyte subpopulations and the mechanisms by which they stimulate and inhibit hematopoiesis. Specifically, we plan to determine the effect of T-cells and T-cell conditioned media on the growth of red cell colonies in vitro and determine their ability to promote marrow engraftment in lethally irradiated canine recipients. In addition, we will identify cells from transfusion sensitized dogs and patients that are responsible for inhibition of in vitro erythropoiesis and determine their sensitivity to radiation and ATG; and in the case of human cells whether or not they can be separated on the basis of size into a subpopulation of T-cells with specific surface receptors. Patients with aplastic anemia will be studied for two purposes; 1) to determine whether or not in vitro inhibition of donor cells by T-cells from transfused patients can predict the fate of subsequent marrow transplantation and 2) whether or not growth of patient derived erythroid colonies after T-cell depletion of peripheral blood mononuclear cells can predict autologous recovery after high dose ATG therapy.